Field of the Invention
The present invention relates to a hydraulic drive system.
Background Art
Work machines, such as a hydraulic excavator or a wheel loader, are equipped with hydraulic cylinders. Hydraulic fluid discharged from a hydraulic pump is supplied to the hydraulic cylinder through a hydraulic circuit. For example, Japanese Laid-open Patent Publication No. 2002-54602 describes a work machine equipped with a hydraulic closed circuit for supplying hydraulic fluid to the hydraulic cylinders. Kinetic energy and potential energy of the members driven by the hydraulic cylinder are regenerated due to the hydraulic circuit being a closed circuit. As a result, fuel consumption of a driving source for driving the hydraulic pump can be reduced.
FIG. 12 illustrates an example of a conventional hydraulic circuit for driving a hydraulic cylinder 103. The hydraulic cylinder 103 includes a cylinder rod 103a and a cylinder tube 103b. The inside of the cylinder tube 103b is partitioned by the cylinder rod 103a into a first chamber 104 and a second chamber 105. The first chamber 104 is connected to a first hydraulic pump 101 via a first flowpath 106. The second chamber 105 is connected to the first hydraulic pump 101 via a second flowpath 107. In this way, the hydraulic cylinder 103 and the first hydraulic pump 101 are connected by a closed circuit. The hydraulic cylinder 103 expands due to the supply of hydraulic fluid to the first chamber 104 and the exhaust of hydraulic fluid from the second chamber 105. The hydraulic cylinder 103 contracts due to the supply of hydraulic fluid to the second chamber 105 and the exhaust of hydraulic fluid from the first chamber 104.
The pressure receiving area of the cylinder rod 103a on the second chamber 105 side is smaller than the pressure receiving area on the first chamber 104 side because the cylinder rod 103a is disposed to pass through the second chamber 105. Therefore, the amount of hydraulic fluid supplied to the first chamber 104 during the expansion of the hydraulic cylinder 103 is greater than the amount of hydraulic fluid exhausted from the second chamber 105. Further, the amount of hydraulic fluid supplied to the second chamber 105 during the contraction of the hydraulic cylinder 103 is less than the amount of hydraulic fluid exhausted from the first chamber 104. Accordingly, the first hydraulic pump 101 and a second hydraulic pump 102 are both disposed in the hydraulic circuit. During the expansion of the hydraulic cylinder 103, the hydraulic fluid discharged from the first hydraulic pump 101 and the second hydraulic pump 102 is supplied to the first chamber 104, and the hydraulic fluid exhausted from the second chamber 105 is recovered by the first hydraulic pump 101. During the contraction of the hydraulic cylinder 103, the hydraulic fluid discharged from the first hydraulic pump 101 is supplied to the second chamber 105, and the hydraulic fluid exhausted from the first chamber 104 is recovered by the first hydraulic pump 101 and the second hydraulic pump 102. In this case, the first hydraulic pump 101 and the second hydraulic pump 102 are controlled so that the ratio between the total discharge flow rate and the discharge flow rate of the first hydraulic pump 101 matches the ratio between the pressure receiving area of the first chamber 104 and the pressure receiving area of the second chamber 105. The total discharge flow rate is the sum of the discharge flow rate from the first hydraulic pump 101 and the discharge flow rate from the second hydraulic pump 102. For example, if the pressure receiving area ratio between the first chamber 104 and the second chamber 105 is 2:1, the first hydraulic pump 101 and the second hydraulic pump 102 are controlled so that the ratio between the total discharge flow rate and the discharge flow rate of the first hydraulic pump 101 is also 2:1. In other words, the first hydraulic pump 101 and the second hydraulic pump 102 are controlled so that the ratio between the discharge flow rate of the first hydraulic pump 101 and the discharge flow rate of the second hydraulic pump 102 is 1:1.